1. Field of the Invention
The present invention relates to an improved optical transmission fiber suitably used for optical communication.
2. Description of the Prior Art
As optical communication systems employed in the case of communicating using coherent light such as a laser, there are a beam guide system, a space transmission system, and a system using an optical transmission fiber. The optical transmission fiber of this invention belongs to the type used in the last system among the aforesaid three optical communication systems.
The optical transmission fibers which have been proposed at present are illustrated in FIG. 1 of the accompanying drawings, and includes (a) a clad type multi-mode optical fiber, (b) a clad type single-mode optical fiber, (c) an O-guide type optical fiber, (d) a self focussing type optical fiber, (e) a single material type optical fiber and combinations of them, for example, (f) a clad type having, in part, gradually varying refractive index and (g) an O-guide type of the similar construction. FIG. 1 shows, respectively, cross sectional view and the refractive index distribution of each of the types.
As the material for optical transmission fibers, glass and in particular those comprising quartz glass as a basic component are preferable from the point of optical loss at present.
However, an optical transmission line composed of a glass can not be used as it is for practical purposes as an optical transmission line (referred to hereinafter as an optical glass fiber) for the following reasons.
A. Firstly, in the optical fiber made of a glass, the elongation at break is quite low, the bending radius at break is quite large, and also it is quite low in strength to impact shock and twisting. Furthermore, although the tensile strength thereof can be rather high on the average but much derivations in the tensile strength exist and the minimum value of the tensile strength is quite low. Therefore, such an optical fiber has a large fault in that the fiber is, as whole, very fragile, is poor in flexibility, and will be broken even by a slight deformation. Moreover, during the manufacturing of the optical fiber or in the course of handling it thereafter, defects such as fine cracks are easily formed on the surface of the fiber and thus if an external force or deformation is applied to the defect-bearing portion, stress is concentrated upon the defect portion, which results in an acceleration of the tendency of the fiber to break. Accordingly, such an optical fiber is not resistant to the external force or deformation occurring during cable production using the optical glass fiber individually or by combining a pluraity of them or during the laying operation of the cable of the optical glass fibers.
B. Secondly, the optical glass fiber used for optical communication must be very transparent, that is, the sources which may cause absorption and scattering of light must be removed from the material as completely as possible. Defects such as fine cracks formed on the surface of the optical glass fiber causes scattering of leaking light, which results in adverse influences to a large extent on the optical communication.
C. Thirdly, if leaking light is totally reflected at the outermost surface of an optical fiber and returns to the central core or if light enters the central core from the outside, noise occurs in the optical communication and thus it is desired to minimize the leaking light from returning to the inside of the optical fiber by leading the leaking light to the outside by some means or to prevent the entrance of external light into the inside as much as possible.
D. Fourthly, for example, in the aforesaid clad type optical fiber shown in FIG. 1, it is impossible to focus or concentrate the light energy completely in core 1 and the rest of the light energy enters clad 2. If core 1 is an ideally transparent body to light, no difficulties occur. However, usually small impurities are present in core 1 or the interface between core 1 and clad 2 is slightly uneven, which causes scattering of light. Such scattered light causes light to leak. In using a single optical glass fiber, such difficulty can be avoidable by releasing the scattered light into surrounding space but in making a cable using a plurality of optical glass fibers, the scattered light can enter the adjacent optical glass fibers and thus the scattered light must be trapped.
Although optical glass fibers have very excellent optical transmission properties, it is impossible to put them to practical use because of the foregoing disadvantages (A), (B), (C) and (D).
An object of this invention is therefore to provide an optical transmission glass fiber which is not accompanied by any of the disadvantages (A), (B), (C) and (D) as described above.